Prior Art Thermal Modulation
Thermal modulation is a means of producing chemical pulses of short duration in capillary gas chromatographic columns.
Thermal modulators grew out of prior art ohmically heated cryogenic traps, which received attention in the scientific literature for some years, following demonstration by Hopkins and Pretorious that ohmic heating of cryogenic traps was faster than the heating obtained with a hot gas stream. (B. J. Hopkins, and V. Pretorious, Journal of Chromatography, 158 (1978) 471). A number of ohmically heated single stage thermal modulators were reported, examples of which are described in the following publications, incorporated in their entireties herein by reference:                1. J. Phillips, et al. “Thermal Desorption Modulation as a Replacement for Sample Injection in Very-Small-Diameter Gas Chromatography Capillary Columns”, Journal of Chromatographic Science 1986, vol. 24, pp. 396-399.)        2. S. Springston. “Cryogenic-focussing, ohmically heated on-column trap for capillary gas chromatography.” Journal of Chromatography, 517 (1990) 67-75.        3. A. van Es, J. Janssen, C. Cramers, and J. Rijks. “Sample Enrichment in High Speed Narrow Bore Capillary Gas Chromatography”, Journal of High Resolution Chromatography and Chromatography Communications, 11 (1988) 852-857.        
Single stage modulators, such as those described in the above publications, were found to concentrate and release sample substances as sharp chemical pulses, but suffered certain limitations. First, ohmic films having very low thermal inertia had to be overdriven in order to heat underlying capillary column segments having much higher thermal inertias. Overdriving caused ohmic coatings to burn out at unpredictable times. A further limitation of the above designs was undesirable tailing of the concentration pulses generated, which limited the utility of the devices for sampling continuous, or semi-continuous sample streams, such as the effluent of an analytical GC column.
The tailing observed with single stage thermal modulators was eliminated by the two-stage thermal modulator introduced by Phillips and Liu, as described in U.S. Pat. Nos. 5,135,549 and 5,196,039, and European Patent No. 0522150, which are incorporated in their entireties herein. Two-stage thermal modulators produced sharp and symmetrical chemical pulses by refocusing a chemical pulse emitted from a first modulator stage at the head of a second thermal modulator stage downstream of the first. The two modulator stages are pulse-heated and cooled 180° out of phase with one another, in order to achieve the refocusing effect. The device proved its ability to sample semi-continuous sample streams in a capillary tube, such as the effluent from an analytical column. This functionality gave rise to the startling advance of comprehensive two-dimensional gas chromatography. As originally implemented, however, ohmically heated two-stage thermal modulators burned out frequently, and unpredictably, and were moreover difficult to prepare and handle.
Ledford and Phillips introduced a solution to the burnout problem, as described in U.S. Pat. No. 6,007,602, which is incorporated in it entirety herein. Although their heater system was reliable, their implementation employed moving parts in the vicinity of the column, which made the device difficult to manufacture and handle in the field. A moving cooler system attributable to Marriott (see for example R. M. Kinghorn and P. J. Marriott, “Enhancement of Signal-to-Noise Ratios in Capillary Gas Chromatography by Using A Longitudinally Modulated Cryogenic System”, Journal of High Resolution Chromatography, 21 (1998) 32-38) suffered similar disadvantages.
Ledford et al eliminated problems associated with moving parts in the vicinity of the column by introducing a two-stage thermal modulator employing pulsed heated and cooled gas jets, described in U.S. Provisional Patent Application No. 60/175,727, filed Jan. 12, 2000, and PCT application WO 01/51170 PCT/US01/01065, filed Jan. 12, 2001, which are incorporated in their entireties herein by reference. The jet modulator was relatively easy to manufacture and use, and produced excellent thermal modulations, including the surprising ability to modulate volatile substances, such as methane. The principle drawback of this design was the complexity of the apparatus, which employed four valves, a heat exchanger, and a bulky mechanical assembly for positioning a modulator tube in the paths of pulsed hot and cold jets.
A variation of the jet modulator was introduced by Beens (J. Beens, et al. “Simple, non-moving modulation interface for comprehensive two-dimensional gas chromatography ”Journal of Chromatography A, 919 (1) (2001) pp. 127-132, which is incorporated in its entirety herein by reference.) Beens employed two high pressure valves to pulse jets of liquid carbon dioxide onto two portions of a capillary tube in the manner known to effect two-stage thermal modulation. The jets were separated by about ten centimeters within the GC oven. Unlike the device of Ledford et al, Beens did not employ gas jets to heat the cooled stages of the modulator tube, but rather relied on the stirred oven bath of the gas chromatograph to heat the modulator stages. To this end, Beens positioned the column on a sprung metal bracket carrying conventional column fittings. This bracket tensioned the modulator tube, held it in the paths of the CO2 jets, and was an open structure that exposed the modulator tube to the oven bath. When applied to comprehensive two-dimensional gas chromatography, Beens's system generated high quality GC×GC images.
Even with the admirable simplicity and good performance of the Beens design, certain limitations were encountered. First, liquid carbon dioxide refrigerant employed in the cold jets produces jet gas temperatures of about −77° C., unsuitable for modulation of chemical compounds with volatilities greater than that of octane. This is problematical for important samples such as gasoline and naptha, in which modulation over the C5+ range, or lower carbon numbers, is desirable. Second, the carbon dioxide consumption rate of the jets was high enough (c.a. 200 std. liters/min, semi-continuous) to pose safety risks in the event of ventilation failure in the room. Third, dead volume between the valves and the jet nozzles could be cleared rapidly only at high gas flow rates. At low gas flow rates, it would be questionable whether the dead volumes would clear rapidly enough to permit high quality thermal modulation. Thus the Beens device requires fairly high gas flow rates through the cold jets. Fourth, high pressure valves present risks to operators that low-pressure valves do not, and are moreover expensive. Fifth, carbon dioxide was admitted to the modulator tube by means of precision fabricated nozzles, which were artful to construct. Sixth, the observed chemical pulse width generated by the Beens device was on the order of 60 to 70 milliseconds, as compared to 36 milliseconds with systems employing hot jet heating of the modulator stages. Narrow pulse widths are desirable in thermal modulation, because well-focused chemical pulses translate to improved sensitivity and resolution in gas chromatographic instruments. Seventh, permanent frost spots appeared on the capillary columns when the cold jets were pulsed at high frequencies, indicating that the heating rate provided by an ambient oven limits the frequency at which the modulator could operate. High frequency modulation is desirable in some applications, such as sensitivity enhancement of one-dimensional gas chromatography, or high speed GC×GC. Eighth, threading columns through a pair of fittings doubled the work of installing columns into the GC oven.
Various embodiments of the prior art are taught, for example, in U.S. Pat. No. 5,135,549 to Phillips et al., printed on Aug. 4, 1992, U.S. Pat. No. 5,196,039 to Phillips et al., printed on Mar. 23, 1993, U.S. Pat. No. 6,007,602 to Ledford et al., printed on Dec. 28, 1999, and U.S. patent application Ser. No. 09/760,508 to Ledford et al., filed on Jan. 12, 2001, which are hereby incorporated in their entireties by reference.
In view of various limitations of prior art thermal modulators, this inventor believed that further innovation in jet modulator technology was needed.